Recent requirements for greater communication capacity stimulate the development of optical wavelength division multiplexing communication systems (WDM systems) using a plurality of optical wavelengths. The optical wavelength division multiplexing communication systems are required to equalize the levels of individual wavelength signals from the viewpoint of nonlinearity suppression and crosstalk suppression. At present, to achieve the level equalization, planar lightwave circuit type variable optical attenuators are about to be used widely. Since the planar lightwave circuit type variable optical attenuators can be easily integrated such as by arraying, they have an advantage over bulk type, magnetooptics type, or MEMS (Micro Electro Mechanical System) type variable optical attenuators from the perspective of economic or miniaturization.
A planar lightwave circuit type variable optical attenuator will be described with reference to the accompanying drawings. FIG. 8 is a plan view showing a typical conventional planar lightwave circuit type variable optical attenuator. The planar lightwave circuit type variable optical attenuator 100 has input waveguides 101a and 101b, a first optical coupler 102, two arm waveguides 103 and 104, a phase controller 105 placed on the arm waveguides, a second optical coupler 106, output waveguides 107a and 107b, and a thin film heater 108. The reference numeral 110 designates a stress-releasing groove, which will be described later.
FIG. 9 is an enlarged sectional view taken along the line IX-IX of FIG. 8 when a conventional example having no stress-releasing grooves 110 is supposed. As shown in FIG. 9, the planar lightwave circuit type variable optical attenuator 100 employs a silicon substrate 109 having excellent thermal conductivity as its substrate, and has a structure in which the thin film heater 108 is placed on the surface of the embedded silica-based waveguides 103 and 104.
The operational principle of the planar lightwave circuit type variable optical attenuator 100 will be described briefly. The light entering the input waveguide 101a is divided into two parts through the first optical coupler 102, and they are fed to the two arm waveguides 103 and 104. The light beams traveling through the arm waveguides 103 and 104 having the phase controller 105 are combined again through the second optical coupler 106. In the course of this, they interfere with each other so that the light is output from the cross port output waveguide 107b when their phases are in phase, from the through port output waveguide 107a when their phases are out of phase by an amount π with each other, and from both of the two output waveguides 107a and 107b in accordance with their phase difference when the phase difference is between zero and π. The phase relationship between the two light beams when they enter the second optical coupler 106 is controlled by the phase controller 105 placed at the arm waveguide 104. As the phase controller 105, a thermooptic phase shifter is often used which is composed of the thin film heater 108 placed on the silica-based waveguides 103 and 104. Since the thermooptic effect is a phenomenon that has no polarization dependence theoretically, it has a characteristic of having a smaller polarization dependence than the electrooptic effect or photo-elastic effect.
As described above, since the conventional planar lightwave circuit type variable optical attenuator utilizing the thermooptic effect can facilitate integration such as arraying, it has an advantage over the variable optical attenuator utilizing other technology such as the electrooptic effect or photo-elastic effect from the standpoint of cost and size reduction.
In practice, however, the conventional planar lightwave circuit type variable optical attenuator utilizing thermooptic effect has a problem of increasing the polarization dependent loss (PDL) when the attenuation of the variable optical attenuator is increased. FIG. 10 illustrates relationships between the optical attenuation and PDL of the variable optical attenuator with the cross-sectional construction in FIG. 9. As illustrated in FIG. 10, a large PDL of nearly 4 dB occurs at the optical attenuation of 15 dB. The large PDL at the optical attenuation offers a serious problem in the operation of a current optical communication system that does not specifies the polarization state in an optical fiber. It has been the greatest factor of preventing the planar lightwave circuit type variable optical attenuators from spreading.
Thus, the conventional planar lightwave circuit type variable optical attenuator has a problem to be solved in that the optical attenuator has a large polarization dependent loss when the optical attenuation of the variable optical attenuator is increased.
Non-patent document 1: Y. Inoue et al., “Polarization sensitivity of a silica waveguide thermo-optic phase shifter for planar lightwave circuits”, IEEE Photon. Technol. Lett., vol. 4, no. 1, pp. 36-38, January 1992.
Non-patent document 2: KIM et al., “Limitation of PMD Compensation Due to Polarization-Dependent Loss in High-Speed Optical Transmission Links”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 1, January 2002.